Many organic high polymers (i.e. large molecules, typically greater than about 10,000 number average molecular weight, composed of repeat units of low molecular weight species, for example, ethylene, propylene, butene, octene, butadiene, styrene and combinations thereof) have surface characteristics that render them difficult to bond to adhesives, inks, and coatings, using conventional bonding agents and methods. Examples of these polymers of low surface energy are polyethylene (PE), polypropylene (PP), ethylene-propylene-diene terpolymer (EPDM) and ethylene/.alpha.-olefin copolymers. Various proposals have been made to overcome these disadvantages.
For example, compositions of styrene-ethylene/butylene-styrene block copolymers and acrylic polymers in a solvent mixture for priming polymers of low surface energy are known. These compositions improve the bonding of pressure sensitive and structural adhesives to such polymers.
Also, it is known that the receptiveness of rubbers (e.g., styrene/butadiene copolymers (SBR) used for shoe soling) and other solid high polymers toward bonding with adhesives (e.g., solvent-based polyurethane and polychloroprenes) is improved by the chlorination of the surface with solutions of halogen donors, such as trichloroisocyanuric acid and N,N-dichlorobenzene sulfonamide. This can be done, for example, by incorporating the halogen donor into a primer or the adhesive itself Typically, however, solutions containing only halogen donors are effective only on substrates containing a high level of ethylenic unsaturation. Also, adhesive compositions, such as polyurethane adhesives, containing a halogen donor have limited stability.
Primer compositions and adhesive compositions (e.g., hydroxy-terminated polyurethane adhesives) that display improved adhesion to organic high polymers having low levels of ethylenic unsaturation are known. Such primer compositions typically contain halogen donors (e.g., dibromodimethylhydantion and trichloroisocyanuric acid) and aromatic isocyanates (i.e., compounds in which isocyanate groups are directly attached to an aromatic carbon, such as in 4,4'-diphenylmethane diisocyanate). See, for example, British Patent Application Nos. 1,458,007 (published Dec. 8, 1976) and 1,460,043 (published Dec. 31, 1976). However, such compositions typically have shelf-lives of less than about 7 days. Thus, they cannot be readily shipped as a one-part system because they must be mixed shortly before use. In addition, the primed substrate will yellow and discolor on exposure to UV radiation due to the presence of the aromatic group in the isocyanate compound. This is undesirable, for example, on white or clear rubber stock.
Compositions of a halogen donor such as trichloroisocyanuric acid, an aliphatic isocyanate-containing compound and an organic solvent are known (Groves, U.S. patent application Ser. No. 08/725,637, filed Oct. 8, 1996) for improving adhesion of adhesives, inks, coatings, and the like, to substrates. These primer compositions are particularly effective on organic polymeric substrates containing some degree of ethylenic unsaturation such as polybutadiene, polyisoprene, natural rubber, styrene-butadiene copolymers, polychloroprene, and the like. However, substrates that contain low levels of, or no, ethylenic unsaturation such as ethylene-vinyl acetate copolymers (EVA), ethylene-propylene-diene terpolymer (EPDM), polypropylene (PP), polyethylene (PE), and ethylene/.alpha.-olefin copolymers do not typically bond well to adhesives, inks, coatings, and the like, although the above-identified compositions of U.S. patent application Ser. No. 30 08/725,637 do provide improved adhesion. For such substrates, mechanical roughening of the surface prior to the application of these primer compositions may be required. However, abrasion of the substrates is not economical and is not always easy or convenient. Thus, there is still a need for primer compositions and application procedures that will even more effectively prime a variety of different substrates, including polymers with little or no ethylenic unsaturation without abrasion. In addition, such primer compositions should possess a long shelf-life and, after application, be stable to UV radiation and high temperature and humidity.